A large number of plastic bottles and similar plastic containers used nowadays are produced in a stretch blow-molding process. In exemplary methods, a so-called preform, which has an elongated tube-like shape on its one longitudinal end, has a bottom and on the other longitudinal end a neck region with molded threaded sections or the like, is inserted into the mold cavity of a blow mold and is inflated by a medium which has been blown in with overpressure. Here the preform is additionally stretched in the axial direction with a stretching mandrel which has been inserted through the neck opening. After the stretching/blow-molding process the finished plastic bottle is removed from the blow mold.
The single-layer or multi-layer preform is produced before the stretch blow-molding process in a separate injection molding method or in a plastic flow molding method. In the so-called single-station stretch blow-molding process the preform is inflated into a plastic container immediately after its production and stretched. But often the plastic containers are produced separately in space and time from the stretch blow-molding process in a two-station method and are temporarily stored for later use. In the later stretch blow-molding process the preforms are heated again in order to produce plastic bottles from them. In this way the two processes, injection molding and stretch blow-molding, can be performed separately and optimally. In the stretch blow-molding process the preform is brought to the specified temperature for example by infrared radiation and is stretched in the axial direction with a stretching rod during the forming process and is radially molded in the molding tool by overpressure.
The raw material for producing plastic bottles in a stretch blow-molding process is mainly polypropylene or PET (polyethylene terephthalate). Polypropylene and PET have been tested many times and their properties have been known for a long time. But due to its low stiffness polypropylene can have relatively poor top load properties. The creep properties of oriented polypropylene are also poor.
Known plastic bottles which are produced in a two-station method, due to the PET which is conventionally used and due to their wide range of application, can engender major difficulties in the recycling stream. Straight plastic containers which are used for example as milk bottles, for preparing cosmetics or for storage of detergents are to be separated in recycling since they are undesirable for direct contact with food. Due to the density of the PET, these bottles however cannot be separated in a floating-sinking process.
For technical and economic reasons it would therefore be desirable if other plastics which are matched to the specific problem, for example HDPE (high density polyethylene) could be processed in a stretch blow-molding process. HDPE also has relatively high stiffness even at low wall thicknesses.
In the injection blow-molding process and in the single-station stretch blow-molding process, the use of HDPE as the raw material which is known from the extrusion blow-molding process for producing plastic bottles is already a popular practice. In the two-station stretch blow-molding process with the high stretching ratios which are used there, HDPE has not been used to date since the poor stretch compaction of the HDPE either did not enable any bottle production at all or resulted in plastic bottles with an overly dramatically fluctuating process or a nonuniform wall thickness profile. Issues also arise fundamentally in injection blow-molding and in a single-station stretch blow-molding process with HDPE. But this can be managed by the preforms being allowed to expand only very little during inflation in order to solve issues with unstable wall thickness. The preforms used in injection blow-molding and in the single-station stretch blow-molding process therefore already have a length which differs only a little from the length of the bottles which have been produced. Accordingly the longitudinal stretching ratio is only 1 to 1.8. The diameter stretching ratio is between 1 and at most 2.2.
The material distribution in known stretch blow-molding can be influenced via the viscosity of the raw material used. But those viscous raw materials which can be easily processed in the stretch blow-molding process often have a viscosity which is too high for the injection molding process. Raw materials which are to be suitable for the two-station stretch blow-molding process should however satisfy the specifications of the injection molding process for producing the preforms and those of the stretch blow-molding process in which plastic bottles are produced from the preforms.